In-vivo imaging of small animals to investigate biological functions, particularly those related to cancer, has become commonplace in the past few years. The advent of imaging systems such as the Concorde MicroPET, MicroSPECT, ImTek MicroCAT, MR, Xenogen IVIS optical imaging systems, and others specifically designed to image small animals, has resulted in a substantial increase in research of animal models of disease. Currently, each of these systems is designed and built independently. As such, each manufacturer has built its own stage for mounting animals or animal holders to its imaging system. Hence, there is currently no common platform for imaging animals in multiple systems without moving the animals to separate, proprietary stages.
One of the main strengths of in-vivo imaging is the ability to image the same animal repeatedly over time or in different imaging devices and accurately compare the images. When an animal is used only for a single data point in a test, trends over time become more difficult to detect, as there are frequently individual differences between the animal subjects. Thus, substantially more data must be collected. Testing a single animal multiple times would thus increase the ease and efficiency of such testing. Data analysis of the resulting images is aided by reproducibly positioning the animal, such that the orientation is consistent across all the images. Current positioning systems, such as taping an animal in place or holding its head with toothbars and ear plugs, do not adequately provide such reproducible positioning of the animal's body.
Positioning is also important in certain imaging systems to ensure that all of the animal is contained within a particular space, and that the animal is centered within the field of view. Some imaging devices, such as CT, are subject to considerable artifacts in the resulting images if any part of the animal extends outside the field of view.
Combined or fused images are images formed by merging several images of the same subject taken at different times or using multiple imaging systems. Such fused images can be very beneficial for a researcher to review multiple biological structures, which may be visible in one imaging method but not another or for viewing changes in an animal over time. Alignment of the images when using multiple imaging systems is essential, therefore the animal must be held immobile during the entire imaging process.
To compare separate images over time or to create fused images, data acquired from an image is typically measured in a device-specific coordinate system, which must be translated to a common coordinate system to compare with data from other imaging devices or sessions. This procedure is called “registration.” Accurate registration depends on knowledge of both orientation of the subject and its location within the imaging system. Since three dimensional objects, such as small animals, can be placed inside an imaging device in innumerable orientations, registration presents a difficult problem.
Several types of registration are known in the art. Software registration employs software to track and correlate either landmarks on the subject or redundant data detected in the subject, such as an eye. External markers called fiducials fixed on the animal can also be used. These markers, however, may move relative to animals and create inaccuracies in the software image registration. Software registration is also limited in that only small changes in orientation can be corrected for. Software registration can be expensive, inaccurate, and time consuming, but is frequently used in small animal imaging for lack of an effective alternative. Additionally, there may be insufficient data available in one or more images for software methods to properly operate, as in the case when only a spherical tumor and nothing else is visible.
Hardware registration, such as tracking the location of fiducial marker on hardware relative to the location of the animal is also used in some systems. The relative positioning of the fiducial to the animal, however, cannot typically be determined with sufficient accuracy when the orientation of the subject is changed slightly between sessions or devices, so hardware registration is typically not possible with multiple imaging sessions in small animals.
Another problem with current imaging systems is that animals are exposed to pathogens. Research using small animals has increasingly utilized various types of transgenic and immuno-compromised animal models. Currently, the imaging systems used with these animals do not offer any type of pathogen barrier to shield the animals from pathogens in the open air.
In-vivo imaging of live animals usually requires that the animals remain motionless during the image acquisition process. For most imaging experiments using small animals, this requires the animal to remain stationary for 10-60 minutes. Safe levels of injected anesthetics typically last only 30-50 minutes, and may not be suitable for longer experiments, or for experiments where two or more imaging systems are used to image the animal and exactly the same positioning is desired. Injected anesthetics also suffer from a variable depth of anesthesia over time, which may affect the biological processes under investigation.
The use of gas anesthetics has become common. Gas provides a constant, easily controlled depth of anesthesia and offers essentially indefinite duration for longer experiments. The use of gas anesthesia is also safer for the animals since it is unlikely the animal will receive an overdose of anesthetic. Recovery times are also very short for gas compared to injected anesthetics, which reduces stress and the amount of time spent in an altered physiological condition. This is particularly important for imaging research where the same animal is frequently imaged, perhaps as often as once per day.
To keep animals alive and healthy for imaging experiments where anesthesia lasts more than a few minutes, it is necessary to keep the animals warm to prevent hypothermia. Without heating, the effects of hypothermia will result in physiological stress or even death to the animals, which is likely to adversely effect uptake and metabolism of injected compounds used for examining biological functions or disease processes. Hypothermia-induced changes are typically not desirable. Therefore, animals are preferably maintained at or near normal physiological temperatures during imaging experiments.
Currently, few systems offer any heating options, and there is not an integrated system available to ensure the animals are kept at normal physiological temperatures throughout the whole imaging experiment process. For microPET research, this is particularly important, since there is often a period of uptake after an imaging agent is injected and prior to image acquisition. If the animal is cold and peripheral blood supply is restricted to maintain core body temperature, there may be little or no uptake into subcutaneous tumors, thus compromising the intended investigation. One option used by some is heating of the air or gas anesthesia. However, this method delivers little heat, due to the low heat capacity of gasses, and when used for extended times can lead to dehydration of the animals.
The creation of disease models in small animals is often a time consuming and expensive process. The complex nature of creating these animal disease models often requires weeks or months of preparation and analysis. Considerable investment in time and money is often spent to create and image these animals. Therefore, the loss of even a single animal can be quite substantial. There is a definite need for equipment and procedures that will aid the collection of imaging data and ensure the health of the animals. In addition, there is a need for ease of use to facilitate high throughput animal imaging to make the most efficient use of time and resources.